Young, run vigorously in protected area (5,4)
I believe the answer is:
green belt
'protected area' is the definition.
(green area which cannot be built on)
'young run vigorously' is the wordplay.
'young' becomes 'green' (green can mean young or inexperienced).
'run vigorously' becomes 'belt' (belt can mean to run fast).
'green'+'belt'='GREEN BELT'
'in' is the link.
(Other definitions for green belt that I've seen before include "Designated zone around town or city, preserved from development" , "Supposedly a no-go area for new building" , "Sign of achievement at judo" , "No build-up in this" , "can't be built on?" .)